Centrifugal compressors achieve compression primarily through the increasing radius of their impellers. High compression can be achieved in a single compact stage, but since the velocities increase continuously through the impeller, supersonic velocities can be experienced at the exit of the impeller, which can increase the losses and decrease the efficiency of the compressor. Axial compressors, on the other hand, typically employ multiple stages of relatively lower compression to cumulatively achieve high compression. A stationary blade row, often referred to as stator vanes, after each rotary blade row reduces the velocity before entering the next rotor. Thus, although total compression increases with each stage, velocity does not increase at the same rate. The result is that multi-stage axial compressors are generally more efficient, for a given pressure ratio, than centrifugal compressors. The trade-off, however is that axial compressors tend to be longer and, in many cases, the compact size and low cost of a centrifugal compressor can outweigh the efficiency penalty, where weight/size are critical, such as in small gas turbine engines in prime mover applications. Nonetheless the trade-off is not always satisfactory, and the desire for better solutions remains.
Although, it has been generally recognized that high efficiency may be realized by providing a series of compression-diffusion stages along the flow path of constant or near constant diameter (i.e. in conventional axial compressors), prior art like U.S. Pat. Nos. 2,350,839 and 4,428,715, and French patent publication 972751A respectively teach employing multiple compression-diffusion stages along the flow path of increasing diameter. Both teach limiting the pressure ratio, radius change and or air speeds in each stage, such that a series of axial stages are provided in what otherwise looks like the envelope of a centrifugal compressor. However, these compressor are relatively complicated and heavy relative to the single stage centrifugal compressor. Such compressors would also tend to exhibit poor surge/stall characteristics if they incorporated into an aero gas turbine engine. Thus, there remains a need for improved efficiency, packaging, weight, cost, durability and/or operability, to name but a few, in compressor/turbine rotor design.